Signal generators that produce multiple output signals with fixed phase relationships between them are known. One such signal generator is a quadrature generator. A quadrature generator is typically utilized to apply the sine and cosine components of a carrier frequency to a pair of mixer circuits in a quadrature amplitude modulator or demodulator.
One known embodiment of a wideband, quadrature generator 100 is depicted in FIG. 1. The quadrature generator 100 includes a clock oscillator 101, a frequency doubler 103, a duty cycle adjuster 105, a duty cycle detector 107, and a divide-by-two circuit 109. The clock oscillator 101 produces a first clock signal 111 at a desired output carrier frequency. The frequency doubler 103 receives the first clock signal 111 and produces a second clock signal 113 at twice the frequency of the first clock signal 111. However, the frequency doubler 103 does not typically produce a clock signal 113 having a 50--50 (i.e., 50%) duty cycle. Thus, the second clock signal 113 is applied to the duty cycle adjuster 105 which, in combination with the duty cycle detector 107, produces a third clock signal 115 at twice the frequency of the first clock signal 111. However, due to collective operation of the duty cycle adjuster 105 and the duty cycle detector 107, the third clock signal 115 has an exact 50--50 duty cycle which is necessary to enable the divide-by-two circuit 109 to produce output signals 117, 119 that are in perfect phase quadrature with one another. The divide-by-two circuit 109 then receives the third clock signal 115 and generates quadrature output signals (I) 117 and (Q) 119.
The divide-by-two circuit 109 is depicted in more detail in FIG. 2. The divide-by-two circuit 109 includes two flip-flops 201, 203 configured in a master-slave arrangement and an inverter 205. The master flip-flop 201 receives at its clock (CLK) input the third clock signal 115 and produces at its output the in-phase (I) quadrature output signal 117. The slave flip-flop 203 receives, at its data input, the I quadrature output signal 117 and, at its clock input, an inverted representation of the third clock signal 115. The slave flip-flop 203 then produces the Q quadrature output signal 119, which is 90 degrees out-of-phase with respect to the I quadrature output signal 117. The Q quadrature output signal 119 is also fed back to the data input of the master flip-flop 201 to provide the symmetry necessary to allow the two flip-flops 201, 203 to produce output signals 117, 119 in exact phase quadrature with each other. More detailed operation of the quadrature generator 100 and the divide-by-two circuit 109 can be found in U.S. Pat. No. 5,375,258.
Although the prior art divide-by-two circuit 109 provides output signals 117, 119 in exact phase quadrature with each other as is optimal for a quadrature generator, the state of the divide-by-two circuit 109 when the clock signal 115 is interrupted is indeterminate. Thus, the phase relationship between the first input signal 111 and the quadrature output signals 117, 119 is not predictable. In practicality, after a clock signal interruption, the divide-by-two circuit 109 has an equal probability of returning to operation in any one of two states. Timing diagrams 300 showing the two equally probably start-up states for the divide-by-two circuit 109 are depicted in FIG. 3. The two equally probable start-up states of the divide-by-two circuit 109 result in quadrature output signals having varying output phases at identical clock times. As shown, the phases of the output signals in state 1 are 180 degrees out-of-phase with respect to the phases of the output signals in state 2. Since either state is equally likely depending upon the time of the clock interruption, any quadrature generator that requires a predictable phase relationship between first input signal 111 and the quadrature output signals 117, 119 would not be able to use the divide-by-two circuit 109 to produce the quadrature output signals 117, 119.
One type of quadrature generator that requires a predictable phase relationship between the first input signal 111 and the quadrature output signals 117, 119 is a quadrature generator used in a Cartesian feedback, linear quadrature amplitude modulation (QAM) transmitter. Such a transmitter employs a first quadrature generator to produce the injection signals applied to the transmitter's forward path upconversion mixers and employs a second quadrature generator to produce the injection signals applied to the transmitter's feedback path down-conversion mixers.
To maintain their linearity, Cartesian feedback transmitters typically "train" their negative feedback loops periodically to insure a 180-degree phase shift around the loop. During each training period, the transmitter opens the feedback loop, conveys a training signal around the loop, measures the phase of the loop, and adjusts the phase of the loop as necessary to obtain the desired 180 degrees of phase shift. Consequently, to maintain the feedback loop phase at 180 degrees after return to normal transmitter operation, the phase changes introduced by the transmitter elements must be substantially the same during and immediately after the training period.
To adjust the loop phase, the phase of the clock signal applied to the down-conversion mixers is altered to achieve the desired 180-degree phase shift around the feedback loop. When training is over, the transmitter will operate normally only if the output phases of the quadrature output signals have not changed substantially since training. However, since the start-up phase of the divide-by-two circuit 109 can produce signals 117, 119 with indeterminate absolute phases, such a circuit, if used, could cause the transmitter's feedback loop to become unstable if the start-up phases of the output signals 117,119 were 180 degrees out-of-phase with their expected phases. For example, if during training, the down-conversion mixers each introduced a ten degree phase change, but then, upon closing the feedback loop, the down-conversion mixer introduced a 190 degree phase change due to the state of the quadrature generator supplying the down-conversion mixers, the loop phase would now be zero degrees instead of 180 degrees, thereby resulting in positive feedback, loop instability, and possible transmitter destruction.
Therefore, a need exists for an apparatus and method for producing a plurality of output signals with fixed phase relationships therebetween that have, at all times, a single, determinate phase relationship with the input signal used to generate them. Such an apparatus and method that could be used to implement a quadrature generator in a Cartesian feedback transmitter would be an improvement over the prior art.